A Trp574–Leu substitution confers resistance to three classes of acetolactate synthase inhibitors in <scp>A</scp> ustralian brome grass ( <scp> <i>B</i> </scp> <i>romus diandrus</i> )

BACKGROUND: Brome grass (Bromus diandrus) is a prevalent weed in southern Australian cropping regions which causes significant economic damage in crops and pastures. Before harvest 2023, two populations (named 6-24 and 7-24) suspected of resistance to imidazolinone herbicides were collected from separate fields. Preliminary screening indicated that these populations were resistant to several different acetolactate synthase (ALS)-inhibiting herbicides. Therefore, further characterisation of the level and mechanism of resistance was carried out. RESULTS: , 2× the label rate for use in imidazolinone-tolerant crops). All examined individuals (15 from each population) carried the same point mutation causing a tryptophan-to-leucine substitution at position 574 in the ALS protein. Application of the cytochrome P450 inhibitor malathion prior to herbicide treatment did not affect survival rates. CONCLUSIONS: This is the first study reporting a Trp574-Leu mutation as the basis for cross-resistance to ALS-inhibiting herbicides (sulfonylureas, imidazolinones and triazolopyrimidines) in Australian populations of B. diandrus. Control of these populations will require strategic use of pre-emergence herbicides and herbicide-tolerant crops in combination with harvest weed seed control. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.